Computed tomography (CT) technology has been in use in the medical field for several years. Such CT scanning instruments produce a cross-sectional view through the subject material along any chosen axis. A two-dimensional X-ray image of electron density variations within the object scanned is produced. The advantages of CT scanning over conventional radiography is found in its much clearer images and its superior density resolution. In medical CT scanners, an X-ray source and a detector array circle a patient in a period of about 2 to 9 seconds and produces an image with maximum resolutions of 0.25 mm in the X-Y plane. This plane can be moved in discrete intervals to obtain information in 3 dimensions. For more details of such medical CT scanners, reference may be made to U.S. Pat. No. 4,157,472 to Beck, Jr. and Barrett (Assignee General Electric Company) and U.S. Pat. No. 4,399,509 to Hounsfield (Assignee EMI Limited).
Many other applications of CT scanning can also be made. For example, in an article entitled, "Computed Tomographic Analysis of Meteorite Inclusions", Science, pages 383-384, Jan. 28, 1983, there is described the non-destructing testing of meteorites for isotopic anomalies in calcium- and aluminum-rich inclusions of heterogeneous materials, such as Allende. The CT scanning equipment described in such article is the Deltascan 2020 from Technicare. In a further application, CT scanning has been applied to the non-destructive testing of wood materials, such as for disease in living trees, see U.S. Pat. No. 4,283,629 to Habermehl. In a yet further application, CT scanning has been applied to the examination of non-living objects, such as motors, ingots, pipes, etc., see U.S. Pat. No. 4,422,177 to Mastronardi, et al. (Assignee American Science and Engineering, Inc.).
More recently, the CT scanning technology has been applied to the field of energy research for examining the interior of stationary or slowly changing earth materials, such as coal, shale and drilling cores. Processes involved in coal gasification and combustion have been monitored using time-lapse CT imagery to observe changes in density (e.g., thermal expansion, fracturing, emission of gases, consumption by combustion) during progressive heating in a controlled atmosphere. Core flooding experiments can now be carried out with CT scanning to aid in enhanced oil recovery and fluid mobility control. For example, the permeability of materials within core samples to various fluids at varying conditions of temperature and pressure can be determined. Such experiments might involve flushing a fluid through a core sample and monitoring the shape of the fluid fronts. By subtracting the images of the cores before and after flooding, the exact shapes of the fluid front was determined. Such core flood experiments allows the interior of the core sample to be observed without disturbing the sample. The sweep efficiency and flow paths of fluids of interest may now be studied on the scale of millimeters. The penetration of X-rays allows experiments to be performed with up to 4 inch diameter core samples.
Drilling fluids can be analyzed by CT scanning as such fluids are characterized by high density brines, various organics and several compositionally different weighting agents. Formation damage can be investigated since CT scanning can detect migration of clays, absorption of organics and the reversibility of completion fluid penetration. Shale oil recovery can be aided as CT scanning could detect penetration by solvents and could directly measure structure changes on retorting. Rock fractures can be identified.